Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). The preferred oxygenate for light olefin production is methanol. The process of converting methanol-to-olefins is called the methanol-to-olefin(s) process. There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Syngas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Syngas is then processed into methanol. Specifically, the components of syngas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol-to-olefins reaction is highly exothermic. Moreover, this reaction has a large amount of water. Water comprises as much as one half of the total weight of the effluent stream to isolate the olefins the effluent stream. Consequently, the water must be removed by condensation in a quench device to isolate the olefin product. The quench device cools the effluent stream to the condensation temperature of water. Quenching the product recovers large quantity of water at the temperature near the boiling point of the quench water. It is desirable to recover heat in higher temperature streams before quenching. Thus, it is one object to recover as much of the heat of the effluent stream before the effluent stream is quenched.
U.S. Pat. No. 6,403,854 is a process of converting oxygenates to olefins with direct product quenching for heat recovery. U.S. Pat. No. 6,403,854 teaches using the reactor effluent stream to cool the effluent stream and superheat the methanol feed stream. Thereafter, the effluent stream is passed to a first stage quench tower of a two-stage quench system.
U.S. Pat. No. 6,121,504 illustrates a process for converting oxygenates to olefins with direct product quenching for heat recovery. According to this patent, the effluent stream may be used to provide heat directly to an oxygenate feedstock. Disclosed is a single heat transfer device for accomplishing this heat exchange between the effluent stream and the oxygenate feedstock.
Nonetheless, there is still a need to recover more heat from the reactor effluent stream in a more efficient manner. The present invention satisfies these and other needs.